In open-loop polar transmitters operating according to modulation schemes such as the Enhanced Data Rate for Global Evolution (EDGE) modulation scheme of the Global System for Mobile Communications (GSM) standard, a power amplifier in the transmit chain is typically non-linear. As a result of the non-linearity of the power amplifier, both Amplitude Modulation to Amplitude Modulation (AM/AM) and Amplitude Modulation to Phase Modulation (AM/PM) distortion are introduced in the system. In order to compensate for the AM/AM and AM/PM distortion of the power amplifier, it is desirable to predistort both amplitude and phase components of a transmit signal prior to amplification by the power amplifier.
One method of predistorting the amplitude and phase components is disclosed in commonly owned and assigned U.S. Patent Application Publication Number 2003/0215025, entitled AM TO PM CORRECTION SYSTEM FOR POLAR MODULATOR, published Nov. 20, 2003; and U.S. Patent Application Publication Number 2003/0215026, entitled AM TO AM CORRECTION SYSTEM FOR POLAR MODULATOR, published Nov. 20, 2003, which are both hereby incorporated herein by reference in their entireties. This method predistorts the amplitude and phase components using predetermined polynomials defining the desired predistortion. For example, coefficients for a third order polynomial defining the desired AM/PM predistortion as a function of the amplitude component may be predetermined. The third order polynomial is then used to provide a phase correction component as a function of the amplitude component which is added to the phase component and corrects the AM/PM distortion of the power amplifier.
The coefficients are typically determined by the manufacturer prior to placing the power amplifier in a mobile terminal, such as a mobile telephone or similar wireless communication device. However, one difficulty with this method is that the AM/PM distortion of the power amplifier changes once the power amplifier is placed into the mobile terminal. Thus, there remains a need for a method of calibrating the coefficients of the polynomial defining the desired AM/PM predistortion after the power amplifier is placed into the mobile terminal. In addition, there also remains a need for a method of calibrating the coefficients of the polynomial defining the desired AM/PM predistortion after the power amplifier is placed into the mobile terminal using existing test equipment.